Prostacyclin (PGI2), a product of cyclooxygenase, has been implicated in the regulation of vascular function, wound repair, inflammatory processes, and acute lung injury. Aerosolized PGI2 induces marked protection against hyperoxic lung injury or lung damage caused by ischemia/reperfusion, and increased levels of PGI2 stable metabolites have been associated with less severe respiratory distress. However molecular mechanisms of PGI2 protective effects on pulmonary endothelial cells (EC) are not well understood. Our published data have shown a reciprocal role for small GTPases Rac and Rho in regulation of endothelial permeability via specific remodeling of actin cytoskeleton and cell contacts. Our preliminary studies strongly suggest barrier protective effects of PGI2 on human pulmonary EC and link them to the Rac-dependent actin cytoskeletal remodeling and enhancement of adherens junctions (AJ) mediated via cAMP-dependent protein kinase (PKA) and novel Epac-Rap1-Tiam1/Vav2-Rac mechanism. Our studies also indicate potent protective effects of PGI2 against thrombin-induced lung EC barrier dysfunction. We hypothesize that PGI2 exerts barrier protective effects on lung EC via activation of PKA- and Epac-mediated signaling leading to activation of Rac- dependent pathways of EC barrier protection via enhancement of peripheral actin cytoskeletal and adherens junctions. We also hypothesize that PGI2 may attenuate acute lung EC barrier dysfunction associated with ventilator induced lung injury (VILI) via inhibition of Rho-dependent pathways of endothelial hyperpermeability. Specific Aim #1 will study PKA- and cAMP/-Epac-Rap1-Tiam/Vav2-Rac-dependent mechanisms underlying PGI2-induced pulmonary EC cytoskeletal remodeling and barrier protection. Specific Aim #2 will study mechanisms of PGI2-induced AJ remodeling associated with PGI2-induced barrier protection and investigate VE-cadherin-mediated regulation of Rac via local recruitment of Epac1, Tiam1, Vav2, Rap1 and Rac. Specific Aim #3 will explore PGI2-induced modulation of Rho signaling underlying protective effects of PGI2 in cell culture model of thrombin-induced pulmonary EC barrier dysfunction and in animal model of ventilator-induced lung injury. We believe that these studies may identify novel protein targets and propose new therapies for prevention of pulmonary vascular barrier dysfunction associated with acute lung inflammation and injury. PUBLIC HEALTH RELEVANCE: Acute respiratory distress syndrome (ARDS) remains a major cause of morbidity and mortality with an overall mortality rate of 30-40%. The acute phase of lung injury is characterized by increased endothelial permeability and compromise of the blood-gas barrier, which allows an influx of protein-rich fluid into the air spaces, causing pulmonary edema. However, despite recent advances in ventilation strategies and a better understanding of the pathophysiology of ALI, there remain few effective treatments for this devastating illness. This application will investigate molecular mechanisms underlying protective effects of prostacyclin against pulmonary vascular endothelial leak induced by edemagenic agonists and pathologic mechanical strain associated with ventilator induced lung injury.